In recent years, there are many chances to capture documents as a camera. In addition, in many case, optical character recognition (OCR) is performed on the captured document image.
Examples of related-art are described in Japanese Laid-open Patent Publication No. 2016-127350 and Japanese Laid-open Patent Publication No. 2010-191952.
If the document is captured by a camera, in some cases, color spots or shadow occur in the background in the captured document image, and thus, a degree of recognition for content such as pictures or characters in the document image is deteriorated.